Durden is part of the split personality of the nameless main character in the movie Fight Club. He is everything the main character wants to be, and has rigged together an operation called Project Mayhem using the main character's body.
As the days go by the nameless main character finds himself going to sleep more and more often, yet his insomnia persists because when he thinks he is asleep, Tyler is actually in possesion of his body, going from city to city setting up fight clubs.
Tyler funds his operation by raiding medical waste facilities and taking liposuctioned fats. He then creates soap using this fat and charges a king's ransom a bar.
When the nameless main character finds out that Tyler is actually a part of him, he shoots himself, killing Tyler in the process. Tyler's last words in the film are: "What's that smell?" This is director David Fincher's reference to the lyricist Ira Gershwin who, while he was dying of an undiagnosed brain tumor, insisted that he could smell burning chicken feathers, and kept asking "what is that smell?" until he fell down dead.